postrm is in binfmtc 0.17-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19  | #! /bin/sh
#remove binfmt
if [ remove = "$1" -a -x /usr/sbin/update-binfmts ]; then
    /usr/sbin/update-binfmts --package binfmtc \
	--remove ccompile /usr/bin/binfmtc-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove cxxcompile /usr/bin/binfmtcxx-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove asmassembly /usr/bin/binfmtasm-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove gcjcompile /usr/bin/binfmtgcj-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove fcompile /usr/bin/binfmtf-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove f95compile /usr/bin/binfmtf95-interpreter
    /usr/sbin/update-binfmts --package binfmtc \
	--remove pcompile /usr/bin/binfmtp-interpreter
fi
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